1. Field of the Invention
The present invention relates to a present tuning apparatus adapted for selecting a desired preset channel. More specifically, the present invention relates to a preset tuning apparatus for selecting a desired preset channel employing a non-volatile analog storage means for storing information concerning a tuning voltage to be applied to a voltage controlled variable reactance device of a tuner.
2. Description of the Prior Art
In reception of the radio broadcasting wave, it is necessary to select a desired broadcasting frequency or channel out of a plurality of radio broadcasting frequencies or channels allotted to different broadcasting stations. A typical prior art preset tuning apparatus adapted for selecting a desired preset channel among a plurality of preset channels is a multi-circuit push-button switch. However, a push-button switch usually makes a circuit through a mechanical touch between paired contacts performed by a manual operation. For this reason, a push-button switch always suffers from a poor electrical contact between the paired contacts caused by deterioration of the contact surfaces.
A tuning apparatus for selecting broadcasting channels or frequencies employing a voltage controlled variable capacitor has also being proposed and put in practical use. Such a voltage controlled variable capacitor utilizes a capacitance formed at a barrier portion of a diode, variable as a function of a reverse voltage applied thereto, and is also known as a variable capacitance diode. In such a tuning apparatus, it is necessary to provide a voltage supply capable of providing a plurality of different tuning voltages in response to a manual operation, each corresponding to a voltage value which causes a capacitance value with which the tuning apparatus selects a desired corresponding channel or frequency.
One type of such a voltage supply for providing a plurality of different voltages comprises a reference voltage source and a plurality of potential dividers for dividing the reference voltage of the voltage source and a switching circuit selectively operable in response to selecting manual operation for withdrawing a corresponding one of the divided voltages. However, such a voltage supply typically comprises a series connection of a plurality of voltage dividing resistors and a switching circuit coupled to each of the junctions of the resistors. Since the resistance value of each voltage dividing resistor determines the output voltage, it is required that the resistance value of each of the voltage dividing resistors be precisely selected. Thus, the above described voltage supply becomes complicated in structure and difficult to implement. In addition, in spite of a recent trend of minituarization of various electronic apparatuses, the above described voltage supply is also difficult to make compact.
A tuning apparatus of interest in connection with the present invention is disclosed in U.S. Pat. No. 3,965,288 issued June 22, 1976; U.S. Pat. No. 3,967,057 issued June 29, 1976; U.S. Pat. No. 3,986,154 issued Oct. 12, 1976 and U.S. Pat. No. 4,008,437 issued Feb. 15, 1977, all assigned to the same assignee as the present invention. These patents describe utilization of an improved non-volatile analog storage means as a voltage source for providing a tuning voltage to be applied to a voltage controlled variable capacitance device included in a tuning apparatus. More specifically, the above referenced patents describe the use of an electrochemical potential memory device by the use of a solid state electrolyte as a non-volatile analog storage means. This electrochemical potential memory device compriese an Ag electrode as a cathode, an Ag-Te alloy electrode as an anode, and a solid state electrolyte having high ion conductivity such as RbAg.sub.4 I.sub.5 sandwiched between both electrodes. When a direct current voltage is applied to the device so that the Ag electrode may be negative, a portion of Ag contained in the Ag-Te alloy electrode migrates over to the Ag electrode, resulting in a decreased activity of Ag in the Ag-Te alloy, and thus an increased potential difference between both electrodes. Thus, this device makes it possible to effect write-in and non-destructive read-out operation and in addition it can hold the memory condition for a relatively longer period of time. Nevertheless, in view of the expensive cost of the above described memory device, it is difficult to employ this memory device in a preset tuning apparatus.
As to be more fully described subsequently, the present invention makes it possible to employ a floating gate type field effect semiconductor memory device as a non-volatile analog storage means for storing information concerning a tuning voltage to be applied to a voltage controlled variable capacitance device employed in a preset tuning apparatus. A floating gate type field effect semiconductor memory device is well known by itself. For example, a typical floating gate type non-volatile semiconductor memory device is disclosed in U.S. Pat. No. 4,004,159, entitled "Electrically Reprogrammable Non-Volatile Floating Gate Semiconductor Memory Device and Method of Operation" and issued Jan. 18, 1977 to the same assignee as the present invention. Typically, the floating gate type non-volatile semiconductor memory device has been utilized as a memory device for storing digital or logical information. It would be extremely advantageous, if a floating gate type field effect semiconductor memory device could be utilized as a non-volatile analog storage means for storing analog information concerning a tuning voltage to be applied to a voltage controlled variable capacitance device employed in a preset tuning apparatus. The present invention is aimed to provide such possibility.